A switched mode power converter is a multi-port network having at least two ports, at least one of which is an input and at least one of which is an output. Inputs absorb electrical power from an external source; outputs deliver electrical power to an external load. The converter is a network of reactive elements, switching elements and, in addition, possibly one or more transformers. The reactive elements include at least one inductor, and possibly one or more capacitors. The switching elements include at least one externally-controlled switch, such as a power transistor, and possibly one or more diode rectifiers. The externally-controlled switches are driven by a control circuit which adjusts the duty cycle of the switches and possibly the timing relationships between various switches so as to regulate the flow of electrical power through the converter. The connection of the switches is such that at least two, and possibly more, topological configurations are assumed by the network as various switches open and close. Each current path in every configuration of the network has no intentional dissipative (lossy) elements, so as to limit power dissipation to the unavoidable minimum caused by the existence of parasitic dissipative losses. Each current path therefore contains inductive reactance, which can be idealized as a lumped inductor, which limits and controls the flow of electrical current. The converter will assume each of its topological configurations in cyclical repetition, as determined by the control circuit.
A switched mode power supply can achieve greater efficiency than a linear regulated power supply because a switched mode power supply uses digital switching instead of power dissipative linear regulation. Reactive elements in a switched mode power supply store energy during the period of time when the digital switching interrupts the power flow.
A typical converter topology is a step down or buck converter. A basic buck converter comprises a capacitor connected between ground and an output terminal, an inductor connected between the output terminal and a switching node, a diode with its cathode connected to the switching node and its anode connected to ground, and a power transistor such as a p-type MOSFET. The drain of the MOSFET is connected to the switching node, the source connects to the input voltage being stepped down, and the gate is connected to a control circuit that switches the power transistor on and off at a frequency typically between 50 kHz and 150 kHz. For clarity, this power transistor can be called a converter power transistor. A feedback circuit provides feedback from the output terminal to the circuitry generating the switching signal allowing the duty cycle of the switching signal to be altered.
Switched mode power converters are classified as to their mode of operation based upon whether the inductor sees continuous or discontinuous current flow. In any given inductor, if there is no finite period of time during which the inductor current is zero, the inductor is said to be operating in continuous conduction mode, or continuous mode. In any given inductor, if there is a finite period of time during which the inductor current stays at zero, the inductor is said to be operating in discontinuous conduction mode, or discontinuous mode. In order for discontinuous conduction to occur, a blocking device must prevent reverse current flow from occurring during a phase of the converter's operation during which the first derivative of the inductor current is negative. In simple canonical converters, containing one inductor, one externally-controlled switch and one diode, the diode acts to block reverse current flow and to allow discontinuous mode operation.
A buck converter can operate in continuous mode or discontinuous mode. When the load is drawing sufficient power, the converter operates in continuous mode, and some positive current is always flowing through the inductor. However, when the power level drops below a predetermined threshold, the current through the inductor will decay to zero and will remain at zero for a finite portion of each cycle of operation. When this condition occurs, the converter is operating in discontinuous mode.
A key indication of the quality of a switched mode power converter is its power efficiency. Although an ideal converter has zero losses, real circuits exhibit numerous loss mechanisms. Power transistors contribute both static losses from the resistance between the drain and the source when the transistor is "on" or conducting, and dynamic losses caused by switching transients. Magnetic components contribute core losses and winding losses. Capacitors contribute ohmic losses due to equivalent series resistance. Diodes contribute both static losses due to the forward voltage drop and dynamic losses due to reverse recovery losses. In modern low voltage switched mode converters, the most objectionable loss is due to the forward voltage drop of the diode, which currently cannot be reduced much below 0.5 volts, even with the best Schottky barrier rectifiers.
To solve the problem of losses due to the diode, switched mode power converter designers commonly substitute a synchronous rectifier for the diode. A general definition of a synchronous rectifier is as follows: A synchronous rectifier is an externally controlled switch which is substituted for a diode rectifier in a switched mode power converter. The switch is either turned on or turned off during each phase of the converter's cycle, so that the synchronous rectifier either appears as an open or a short in each topological configuration assumed by the converter. This implies that the switch is operated in synchrony with at least one other switching element of the converter, and thus the name `synchronous rectifier`. For example, a synchronous rectifier can be a power transistor such as a bipolar power transistor or a MOSFET power transistor. In a buck converter, the synchronous rectifier is switched on during the period of time when the converter power transistor is "off", allowing the synchronous rectifier to source the inductor current. While the converter power transistor is on, the synchronous rectifier is switched off.
A MOSFET is commonly used as a synchronous rectifier. MOSFET synchronous rectifiers are poorly adapted for use in converters that supply widely varying loads, because the MOSFET synchronous rectifier prevents the converter from entering the discontinuous mode of operation.
In a discontinuous mode of operation, there is an interval of time during which a diode would normally block reverse current to prevent back conduction. A conducting MOSFET cannot block reverse current since it conducts in both quadrant one and quadrant three. Therefore, discontinuous operation is prevented. Considerable AC currents circulate in the converter even if little or no power is delivered to the load. Therefore, a converter with MOSFET synchronous rectifiers becomes increasingly inefficient as the output power approaches zero. Circuits which employ bipolar transistors experience similar reverse conduction losses at low power levels.
One popular application for low voltage, synchronously rectified, switched mode power converters is in portable computers. Because of limited battery capacity, portable computers shut down disk drives and displays when not in use. These power conserving techniques result in very wide variations in load (as high as 1000:1). These wide load variations make discontinuous mode operation highly desirable. Because high efficiency is critical, synchronous rectification is often employed and back conduction through the synchronous rectifier at low power levels is unacceptable. Accordingly, an efficient power converter should employ a synchronous rectifier at high power levels and an ordinary rectifier, such as a diode, at low power levels. The efficiency loss due to the forward drop of the diode rectifier is more than compensated by the elimination of reverse conduction losses.
Two existing solutions are known to allow synchronous rectification in wide load power converters. First, some switched mode power converters have a mechanism to manually enable or disable the synchronous rectifier using an externally generated signal. While the synchronous rectifier is disabled, a parallel-connected diode takes over. This approach, however, requires an external circuit to detect the onset of low power operation and manually disable the synchronous rectifier. This solution thus requires additional circuit design and the additional circuitry consumes additional board space. This solution may increase the size and cost of an electronic device employing such a switched mode power converter.
A second existing solution requires one or more external current sense resistors and the onset of low power operation is detected by monitoring the voltage across these resistors to detect current variations. Although this scheme eliminates the need for an external control signal, it has two serious disadvantages. First, the current sense resistors represent unwanted dissipative elements in the power circuit, thus creating an efficiency loss that is significant at high power levels. Second, the current sense resistors are sensitive to noise that may interfere with the proper operation of the circuit. Therefore, using current sense resistors complicates board layout and may represent an unpredictable source of transient instabilities due to unexpected mode transitions. Current sense transformers can be used in place of current sense resistors, but they are relatively expensive and require one or more additional magnetic components to be added to the circuit.